DATA LINK LAYER
A checksum is a type of redundancy check used to detect errors in a data.
Errors that frequently occur in a data when written to a disk, transmitted across a network or even if manipulated. These errors are very small for example for single incorrect bit but such small errors can affect the quality of data and even make it useless. In the simplest way a checksum is created on calculating the binary values in a packet or block of data using some algorithm and storing the results with the same data.
Network layer is getting service from the data link layer .Network layer is interested in getting messages to a corresponding network layer module on an adjacent machine.
This remote Network Layer should receive the identical message generated by the sender .The Network Layer then should be sure that all messages that it sends, will be delivered error free.
The Network Layer demands that messages to be delivered to the remote peer should be of exact order as they are sent.
Due to which the ISO came up with a reference model which suggests that the data link layer would provide services to send the correct message which will be verified by the protocols .
Protocol is of two kind-
- Noiseless channel
- Noise channel
Noiseless channel protocol
Here two paths have been introduced one that uses flow control and the other that does not use any flow control.
It is of two types- simplest and stop & wait protocol.
It is the simplest form of. In this protocol the receiver
indicates that it is ready to receive data for each frame.It works on simple Operations:
1. Sender-Transmits a single frame
2. Receiver: Transmits the acknowledgment (ACK)
Stop and wait protocols
In this case the Frames have 1-bit sequence numbers (SN = 0 or 1) and the Receiver sends an acknowledgement (1-SN) if frame SN is correctly received, then the sender waits for an ACK (1-SN) before transmitting the next frame with sequence number 1-Sn and If sender does not receive anything before a timeout value expires and it retransmits frame SN
Here there are disturbances in each path due to which acknowledgement is required.it is of three types-
- Stop-and-Wait ARQ
- Go-Back-N ARQ
- Selective ARQ
Go-Back-N Automatic Repeat Request
Go-Back-N ARQ uses the sliding window type of protocol. When no errors are found the operations are identical to Sliding Window.
- A station can send multiple frames as allowed by the window size
- Receiver sends a NAK i provided frame i has error. After this the receiver discards all incoming frames unless the frame in error was correctly retransmitted
- If sender receives a NAK i it will retransmit frame I and all packets i+1, i+2,… which have been sent, but not been acknowledged.
Selective repeat request
Similar to Go-Back-N ARQ here the sender only retransmits frames for those which a NAK is received
Advantage over Go-Back-N:
- Less Retransmissions.
- High amount of complexity at sender and receiver
- Each frame should be acknowledged individually (no cumulative acknowledgements)
- Receiver receives frames out of sequence
questions and answers
1. Name the network support layers?
Ans- The network support layers are the data link layer and physical layer.
2.name the user support layers?
ans- the user suppor layer are the session layer,presentation layer and application layer.
ans-combination of many neworks into a large one.
4.What is protocol?
ans- set of rules governing the communication system.
5.What is the use of ISOC?
ANS- promotion of research activities.